Light metal molded part, especially a crankcase for an internal combustion engine

ABSTRACT

Described is a molded part made of light metal, in particular a crankcase for an internal combustion engine, comprising holes ( 5 ) for receiving fastening screws ( 6 ) with the help of which compressive strains can be exerted on the molded body in a light metal area. In order to create advantageous constructional conditions, it is proposed that at least one open-pored sintered body ( 7 ) is cast in the light metal area exposed to compressive strains with infiltration of the light metal at least in the pores of a surface layer; and that the sintered body ( 7 ) having a lower coefficient of thermal expansion than the light metal has a strength at least corresponding with the strength of the light area when combined with the infiltrated light metal.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicants claim priority under 35 U.S.C. §119 of Austrian ApplicationNo. A 497/99 filed Mar. 19, 1999. Applicants also claim priority under35 U.S.C. §365 of PCT/AT00/00047 filed Feb. 23, 2000. The internationalapplication under PCT article 21 (2) was not published in English.

The invention relates to a molded part made of light metal, inparticular to a crankcase for an internal combustion engine, comprisingholes for receiving fastening screws with the help of which compressivestrains can be exerted on the molded body in a light metal area.

As molded parts made of light metal exhibit a different thermalexpansion behavior as compared to the steel-made fastening screwsusually employed, the stress conditions change with the change intemperature because of compressive forces exerted on molded light metalbodies with the help of fastening screws made of steel. This may becomenoticeable, for example in the form of a substantial reduction of theclamping forces. So as to avoid in connection with bearing blocks oflight-metal crankcases any excessive increase in the play of the bearingat higher operating temperatures, it is known (from U.S. Pat. No.5,203,854 A) to support the crankshaft bearings with the bearing screwsin gray cast inserts, which are cast into the walls of the bearing. Thismeans that the expansion behavior is determined by the gray castinserts, which form a substantial part of the bearing walls, and not bythe adjoining light metal areas of the bearing walls, so that largelyuniform conditions of thermal expansion can be expected in the zone ofthe crankshaft bearings and their screw connections. This has anadvantageous effect on the play of the bearing or the strength of thescrew joint. However, it is a drawback in connection with such knowncrankcases, among other things, that the weight advantage of the lightmetal walls is cancelled to a substantial degree because of the graycast insert receiving the screw forces and the geometric dimensionsconnected therewith.

Therefore, the invention is based on the problem of realizing a moldedpart made of light metal, in particular a crankcase for an internalcombustion engine of the type specified above, in such a manner that itis possible to create advantageous fastening conditions in spite of thedifferent thermal expansion behavior of the light metal and thefastening screws, notably by largely exploiting the weight advantagesoffered by the light metal.

The problem is solved by the invention in that at least one open-poredsintered body is cast in the light metal area exposed to compressivestrains into the pores of a surface layer, with infiltration of thelight metal, and in that the sintered body having a lower coefficient ofthermal expansion than the light metal, has a strength corresponding atleast with the strength of the light metal at least after it has beencombined with the infiltrated light metal.

A sintered body with a coefficient of thermal expansion that is lowerthan the one of the light metal is capable of altering the thermalexpansion behavior of the light metal if the sintered body is joinedwith the light metal in a manner proof to shear, and that the compositehas a strength at least corresponding with the strength of the lightmetal, so that the light metal is prevented from freely expanding underheat in the area of the sintered body. The shear-proof joint between thelight metal and the sintered body is obtained if the light metal melt,as it is being cast preferably by a pressure casting method, infiltratesat least the pores of a surface layer of the sintered body. This permitsadequately high shearing strains to build up in the finished molded partin the area of transition from the light metal to the sintered body inorder to suppress any relative movement between the sintered body andthe light metal. The sintered bodies, which can be employed inaccordance with the given stress requirements that the molded part hasto meet, can be provided in this connection with comparatively smalldimensions, so that the increase in weight connected with such sinteredbodies remains limited all the more so because the porosity required forthe sintered bodies results in a weight reduction as compared tonon-sintered inserts. Since the strength of the sintered body increaseswith infiltration of the light metal, the sintered body withoutinfiltrated light metal may have a lower strength than the light metalbut will nonetheless satisfy the strength requirements with respect tolimitation of free expandability of the light metal if the sintered bodyinfiltrated with light metal has the strength required for suchlimitation.

Particularly advantageous construction conditions can be obtained if thesintered body has a coefficient of thermal expansion adapted to thefastening screws, so that comparable conditions are obtained withrespect to the strength of the screwed joint when steel parts arescrewed together.

Since a shear-proof joint between the sintered body and the light metalis primarily involved in the present case by infiltrating the lightmetal into the pores of the sintered body, the core area of the sinteredbody may have a higher density than the one found in the surface area.This facilitates the infiltration of the light metal into the pores ofthe surface layer, on the one hand, and leads to a higher strength ofthe sintered bodies on the other. However, the strength of the cast,sintered bodies can be improved by reinforcements as well. Of course,such reinforcements may not permanently impair the capability of thesintered body of being infiltrated when the light metal is poured aroundthe sintered body.

The object of the invention is shown by way of example in the drawing,in which:

FIG. 1 shows the lower part of a crankcase as defined by the inventionby a cutout in a simplified diagram; and

FIG. 2 shows said lower part of the crankcase by a cross section througha wall of the bearing.

The shown lower part of the light-metal crankcase as defined by theinvention comprises in the conventional manner the outer crankcase walls1 extending lengthwise, and the bearing walls 2 aligned transversely tosaid longitudinal walls and connecting said outer crankcase walls 1.Starting from the area of separation 3 versus the upper part of thecrankcase, said bearing walls form the receptacles 4 for receiving thebearing cups of the crankshaft bearings, said receptacles having asemi-circular cross section. Provision is made on both sides of saidreceptacles 4 for the receiving holes 5 drilled in the bearing walls 2for receiving the fastening screws 6, via which the bearing forces aredischarged into the bearing walls 2. Within the area of the receivingholes 5, the sintered bodies 7, which have an adequate porosity, arecast into the light metal of the bearing walls 2 in order to permitinfiltration of the light metal into the pores of at least one surfacelayer. Such infiltration of the sintered bodies 7 by the light metal isindicated in FIG. 2 by the shaded part of the light-metal bearing walls2 extending into the sintered bodies 7. For saving weight, the sinteredbodies 7 may be provided with the recesses 8.

Because of the infiltration of the sintered bodies 7, the shear strainsoccurring due to the different thermal expansion behavior of the lightmetal and the sintered bodies can be transferred from the light metal tothe sintered bodies, which prevents relative movements from taking placebetween the light metal and the sintered bodies 7. Accordingly, thebearing walls 2 made of light metal adapt themselves to the thermalexpansion behavior of the sintered bodies 7 at least within the area ofthe latter if such sintered bodies 7 are provided with adequatestrength. This means, for example with a required adaptation of thethermal expansion behavior of the bearing walls 2 to the thermalexpansion behavior of the fastening screws 6, that the sintered bodies 7must have a corresponding behavior of thermal expansion in order tolimit the thermal expansion behavior of the bearing walls 2.

In order to facilitate the infiltration of the sintered bodies 7 by thelight metal as the bearing walls 2 are being cast, the sintered bodies 7have to be provided with a corresponding porosity. This can be easilyassured, for example by selecting the grain size of the sinter powderand its distribution accordingly, or by the space keeping meansdetermining the later pores. Furthermore, the sintered bodies maycontain alloying elements, for example nickel, which reactexothermically with the light metal and thus reduce the viscosity of themelt. For securing adequate strength, the sintered bodies may beprovided with a reinforcement as well, which is compressed jointly withthe sinter powder and consists of, for example a wire mesh orreinforcing fibers.

The invention is, of course, not limited to the exemplified embodimentshown but may be employed whereever forces have to be introduced into amolded light-metal part via fastening screws under different thermalstresses. According to the exemplified embodiment, the sintered bodies 7may be arranged next to the holes 5 for receiving the fastening screws6, but also may be penetrated by the fastening screws 7. What mattershere only is that the light metal area of the molded part maintainedunder pressure strain by the fastening screws 6 is prevented from itsfree thermal expansion behavior.

What is claimed is:
 1. A molded part made of light metal, in particulara crankcase for an internal combustion engine, the molded part havingholes for receiving fastening screws which may exert compressive strainson the molded body, comprising at least one open-pored sintered bodycast in the light metal area exposed to the compressive strains, thelight metal infiltrating in the pores of a surface layer of the sinteredbody, and the sintered body having a lower coefficient of thermalexpansion than the light metal and a strength corresponding at least tothe strength of the light metal when combined with the infiltrated lightmetal.
 2. The molded part according to claim 1, wherein the sinteredbody has a coefficient of thermal expansion adapted to the fasteningscrews.
 3. The molded part according to claim 1, wherein the sinteredbody has a higher density in a core area than in the surface layer. 4.The molded part according to claim 1, wherein the sintered bodycomprises a reinforcement.